Accelerometer



July 25, 1967 c. D. BOCK ET AL 3,332,290

ACCELEROMETER Filed May 22, 1956 2 Sheets-Sheet 1 FR EQ BRIDGE vINVENTOPS. CHARLES D. BOCK 7:7 2 PAUL S-QJOEGENSEN 9- JOSEPH STATSINGEEATTOPNE).

United States Patent 3,332,290 ACCELEROMETER Charles D. Bock, FloralPark, N.Y., Paul S. Jorgensen,

Westbury, N..l., and Joseph Statsinger, Bayside, N.Y.,

assignors to American Bosch Anna Corporation, a corporation of New YorkFiled May 22, 1956, Ser. No. 586,615 7 Claims. (Cl. 73-503) The presentinvention relates to accelerometers and has particular reference tointegrating accelerometers.

Integrating accelerometers are used to obtain high accuracy of velocitymeasurement. Present day applications of navigational instruments inhigh speed vehicles and missiles demand extreme accuracy, wide operatingrange and utmost reliability of its acceleration sensing devices. Thepresent invention is a new type of accelerometer employing vibratingmembers as the sensing means and possessing these operatingcharacteristics.

In accordance with this invention, a pair of wires or tapes capable ofsustained vibration in at least one plane are attached to a suspendedweight, and the wires or tapes are stretched between rigid supports toplace them under tension. The weight is supported laterally by centeringmembers symmetrically placed at right angles to the stretched wires ortapes. Permanent magnets provide magnetic fields whereby the wires ortapes are caused to vibrate in planes normal to each other upon passageof alternating current through these wires. In addition, the E.M.F.produced in the wires or tapes as a result of the vibration is effectivein keeping the wires vibrating. An acceleration of the supports in thedirection of the stretched wires will cause the suspended weight toincrease the tension in one wire and reduce the tension in the otherthereby providing means for determining the value of the vehicleacceleration fro-m the resulting vibrating frequency difference.

It will be seen that means must be provided for preventing crosscoupling between the wires through the sensing mass. In the preferredembodiment the wires are decoupled by providing separate sensitivemasses or weights which are joined by a member having a naturalfrequency much higher than either of the sensing wires. When tapes areused, the inherent stiffness in the plane normal to the plane ofvibration is effective in preventing coupling between the tapes.

Since the frequency of wire vibration is proportional to the square rootof the tension instead of being a linear function of the tension, thefrequency difference is only approximately proportional to acceleration.However, the sum frequency which is similarly dependent on the squareroot of the tension provides a means for linearizing the frequencydifference output and thereby determining the velocity of the supportswithout approximation.

For a more complete understanding of the invention, reference may be hadto the accompanying diagrams in which:

FIG. 1 is a pictorial representation of the sensing element;

FIG. 2 is a schematic diagram of the electrical and mechanicalconnections of the sensing element and associated computer;

FiG. 3 is an alternative arrangement for a portion of FIG. 2;

FIG. 4 is a diagram of a particular frequency subtraction device;

FIG. 5 is a diagram of a particular frequency summing device; and

FIG. 6 is a diagram of a particular frequency bridge.

Referring now to FIG. 1 which shows a preferred embodiment of thesensing element of the accelerometer, the electrically conducting wires10 and 11 are connected to the equal and similar weights 12 and 13respectively which are joined by the wire 14 and the entire assembly 10,12, 14, 13 and 11 is connected between the rigid supports of a frame 15so that the wires 10, 11 are put under tension. The ends of the wires 10and 11 are electrically insulated from each other and from frame 15 bythe insulators 16 and 17. If the frame is made of anon-conductor, theinsulators 16 and 17 are not required. Any suitable clamping means maybe employed, but it should be such as to allow initial adjustment in thetension of the wires 10 and 11.

A permanent magnet 18 produces a transverse magnetic field across wire10 and has its pole pieces oriented so that the magnetic field is in thevertical plane in FIG. 1, while a similar magnet 19, has its pole piecespositioned and oriented so as to produce a transverse magnetic fieldacross wire 11 in the horizontal plane in FIG. 1. Passage of alternatingcurrent through the wires 10 and 11 will, therefore, cause wire 10 tovibrate in a horizontal plane and wire 11 to vibrate in a verticalplane.

The weight 12 is suspended in the frame 15 by the symmetrically placedtapes 20 which are arranged so as to allow motion of the weight 12 inthe direction of the axis of wires 10 and 11. Similarly, weight 13 issuspended in frame 15 by the tapes 21.

The wires 10 and 11 are vibrated at their natural frequency by employingtwo oscillator amplifiers 22, 23, FIG. 2, each having one of the wires10 or 11 as the load and as the controlling element in the oscillatorcircuit. Frequency standards employing vibrating wire oscillators ofthis type are not new and need not be further described here, except topoint out that the natural frequency of vibration, f depends on thephysical constants of the vibrating member and is proportional to J2 mlwhere T is the tension of the wire, I is its length, and m is the massper unit length. Under zero acceleration, the wires 10, 11 are identicaland are exposed to identical conditions, so that the natural frequencyof vibration is the same for each wire and is preferably about 5000c.p.s. When the frame 15 is subjected to an acceleration in thedirection of wires 10, 11, to the left in FIG. 1 for example, thetension in one wire 10 increases and in the other wire 11, decreases.This change in tension creates a change in the natural frequency ofvibration according to the relationship It appears that the differencefrequency,

Id=f1n-f11=\ fn fn can be used to determine the accelerationproportional to ka, but this cannot be done readily. Considering thebinominal expansion of (3) it will be seen that the non-linear termresulting from the square root relationship will make the differencefrequency difiicult to interpret in terms of acceleration. Also, thescaling factor f is dependent upon the natural frequency of vibration ofthe wires.

The present invention employs the sum frequency The values f and f arethen added to provide an output f =f +f which by addition of f toEquation 6 is seen to be fo=fd+fd (fs fr) If K is made equal to 1/1 thensubstitution of (3) and into (9) yields Inspection of Equation 10 showsthat the output f is linear in ka, has a scaling factor f which is aconstant value independent of the wires, and which can be maintained toa high degree of accuracy.

A preferred instrumentation of the above is shown schematically in FIG.2. The voltage outputs of the amplifieroscillators 22 and 23 are appliedto the phase splitting networks 24, 25 respectively both of whichproduce a pair of output voltages each being proportional in magnitudeand frequency to the input voltage but differing in phase by 90 fromeach other. The outputs of one such phase splitting network 24 areapplied to the stator windings of a synchronous motor 26, and theoutputs of the other phase splitting network 25 are applied to the rotorwindings of the synchronous motor. The speed at which the rotor of sucha synchronous motor turns is given by the equation 2fa f where n is thenumber of poles on the motor.

The synchronous motor 26 is a special type of motor having a two-phasestator winding and a two-phase rotor winding where the two statorwindings are displaced by 90 electrical degrees and the two rotorwindings are also displaced by 90 electrical degrees. The rotor andstator windings both have the same number of poles. The speed at whichthe motor turns its output shaft 27 is proportional to the frequencydifference between the frequencies of the exciting voltages and thedirection of rotation depends on which of the frequencies is higher. Itshould be pointed out here that the motor 26 must be designed so that itwill never fall out of synchronism nor skip one cycle. Phase-splittingnetworks are well known in the art and may be simply capacitors forexample or may equally well comprise networks containing additionalcomponents.

An alternate method of obtaining the same result is shown in FIG. 3where the shaft 27 is driven by a synchronous motor 28 which isenergized by the voltage of difference frequency, f obtained from theoutput of the frequency discriminator 29 having inputs of h and f Motor28 has a permanent magnet rotor and a single phase stator winding whichis energized by a constant magnitude variable frequency current, thefrequency being equal to the difference frequency between fro and i andnormally varies between 0 and 500 c.p.s. The speed of shaft 27 is thenproportional to the difference frequency f -i or is proportional to Zf/n.

The frequency difference discriminator 29 may be of any desired type, atypical arrangement being shown in FIG. 4 which shows a ring typedemodulator. A voltage of frequency i is applied across one diameter ofa rectifier bridge 60 through a transformer 61 and a second voltagehaving a frequency is applied across the other diameter of the rectifierbridge 60 through a transformer 62. The output of the demodulator, takenbetween center taps on the secondary windings of transformers 61 and 62,is a voltage containing components having frequencies f1o-f1n fro-H11,f10-f1n f10+f1b Since in the present invention, f f is a low frequencywhich may vary between 0 and 500 c.p.s. while f and i themselves areapproximately 5000 c.p.s., the f -f component can be easily separatedfrom the output by a simple low pass filter 63.

In a sum frequency discriminator a basic ring type demodulator andfilter cannot be used alone since the sum frequency component f -Hcannot be separated from 3f f when f and i are nearly the same, andfiltering of f -l-f would be unsuccessful.

One scheme for obtaining the sum frequency is shown in FIG. 5. Thevoltage of frequency f and a reference voltage of frequency f which isvery much higher than f for example f may be ten times as great as 1'are applied to a demodulator 64 and the (f -H component of its outputvoltage is obtained by a band-pass filter 65. This component can besuccessfully filtered out since the frequencies f and f are not nearlyequal, and no voltage having a frequency equal to the sum frequency willbe produced by a difference in the lower harmonics of f and fro- Thevoltage of frequency f and the reference voltage of frequency f areapplied to a second demodulator 66, the output of which is transmittedthrough filter 67 to separate the f f component. The outputs of thefilters 65 and 67 are then applied to a third demodulator 68 to providea voltage having a frequency f equal to the dif' ference in the inputfrequencies, or

which is the sum of the orginal frequencies f and i Other types offrequency summing circuits may be employed if convenient, and theinvention is not to be limited to the embodiment here described.

The output voltage of frequency summing means 30 and the output voltageof the reference voltage supply 31, having a constant amplitude and afrequency of f, are applied to the demodulator 32 which is adapted toproduce an output voltage of constant amplitude and frequency equal tothe difference between the input voltage frequencies or f, +fDemodulator 32 may be of the same type as the difference demodulator 29.

The difference frequency f (f +f is transformed into a shaftdisplacement by the frequency bridge 33 and motor 34. The frequencybridge 33 contains a variable element which is adjusted by the outputshaft 35 of motor 34 until the output of the bridge 33 is zero and theposition of the shaft 35 corresponds to the difference frequency f (f -HThe frequency bridge 33 may be of the type shown in FIG. 6, for example.

Here, the demodulator 32 supplies the difference voltage f,(f +f to ademodulator and filter 36 which is also supplied by a votlage ofvariable frequency from the variable oscillator 37, and which is adaptedto produce a voltage having a frequency equal to the difference in thefrequencies of the oscillator 37 and the demodulator 32 output voltages.The output of the demodulator-filter 36 is supplied to the null network38 which has a characteristic such that the voltage output is zero atsome pre-selected input frequency, and when the input frequency deviatestherefrom the magnitude of the output voltage indicates the magnitude ofthe deviation and the phase of the output indicates the direction of thedeviation. The network 38 may consist of a bridged T network, forexample, having properly chosen components. It will be seen that if theoscillator 37 output frequency is very nearly the same as the frequencyof the demodulator 32 output, then the output frequency of thedemodulator 36 would be a low beat frequency of one or two cycles persecond and unsatisfactory operation might result. For this reason, theoscillator 37 normally produces a voltage at some preselected frequency,for example 400 cycles per second greater than the nominal value of f f+f The null circuit input or output is therefore a voltage having afrequency approximately equal to 400 cycles per second, and a magnitudeproportional to the deviation of the input frequency from 400 c.p.s.

The motor 34 is energized by the output of null network 38 throughamplifier 39 to drive shaft 35 to adjust a variable capacitor, forexample, in the oscillator 37 and thereby to adjust the frequency of theoscillator output. When the oscillator output frequency is such that thenull network input is exactly 400 cycles, the output of the null networkis zero and motor 34 is deenergized. At this point the position of shaft35 corresponds to the frequency of the signal output of the demodulatorfilter 32. It should be remembered that the above is merely one proposedembodiment of a frequency bridge which operates in the required mannerbut that many other types may be used if desired, and the invention isvnot to be limited to this embodiment.

Shaft 27 drives the disc 40 of a mechanical multiplier 41 of theball-and-disc integrator type, the ball 42 of which is displaced along aradius of disc 40 by the shaft 35.-The roller 43 of the multiplier 41 istherefore driven at a rate equal to the product of the speed of shaft 27and the displacement of ball 42, or

fd T [fB fr] where K is the constant of proportionality generated by themultiplier 41.

The rotation of shaft 27 is added to the rotation of roller 43 in themechanical differential 44 to drive shaft 45 at a speed, f equal to:

If now K is chosen equal to 1/1 by proper selection of the mechanicalelements of the multiplier 41 then Equation 12 reduces to Equation 13,which corresponds to that given earlier by Equation Now, since the speedof shaft 45 is proportional to ka, or to the acceleration, thedisplacement of shaft 45 is proportional to the velocity. If the initialvelocity of the craft is not zero, it may be added to the displacementof shaft 45 by the differential 46 to provide a true velocity indicationat the output shaft 47 which may be read on dial 48.

From the foregoing explanation, the theoretical operation of theaccelerometer will be clear. However, some of the physical requirementsfor the various elements which are necessary for making theaccelerometer a practical instrument are discussed below.

In order to keep changes in the relative length of the two wires withinacceptable limits the wires should have a low thermal coefficient ofexpansion and good thermal conductivity. The mechanical strength of thewire material should be high in order to be able to attain a relativelyhigh natural frequency. A high modulus of elasticity means smallerdisplacements and shorter supporting wires and therefore smaller spacerequirements. A low resistivity wire has less power dissipation and isless likely to raise the temperature. All of these requirements anddesirable characteristics are met by a number of materials such asberyllium-copper, tungsten or molybdenum, for example.

It has been found that for the expected range and accuracy it isadvisable to mount the sensitive wire structure in an evacuated envelopein order to preclude the unknown effects of dirt and moisture.

In order to prevent the possibility of operating the wire in anon-linear region, the amplitude of its vibration must be controlled soas not to exceed some given value, which may be determinedmathematically or by experimentation. It appears that the amplitudecontrol is most satisfactorily accomplished through use of a non-linearnetwork between the oscillator and the vibrating wire.

Although the described embodiment has specified wire for the vibratingmembers 10 and 11, it is evident that tapes or ribbons can be employedequally well. In fact, it has been found that the tapes have certainadvantages over wire in the matter of decoupling and reliability. Forexample, wires are known to be anisotropic but tapes can be orientedinto the operating position without difficulty. The tapes are caused tovibrate in planes perpendicular to each other to effect the decouplingmentioned above.

We claim:

1. In a device of the character described, a pair of spaced supports, amass, a pair of coaxial tensioned wires extending between said mass andsaid supports, first electronic means for keeping one of said wiresvibrating at its natural frequency and having an output signalalternating at said one natural frequency, second electronic means forkeeping the other of said wires vibrating at its natural frequency andhaving an output signal alternating at said other natural frequency,means for determining the difference between said natural frequencies,means for determining the sum of said natural frequencies, means formultiplying an amount controlled by said sum by said difference toobtain a product, and means for adding said product to said differenceto obtain an output indicative of the velocity of the supports in theaxis of said wires.

2. In a device of the character described, a pair of spaced supports, amass, a pair of coaxial tensioned wires extending between said mass andsaid supports, first electronic means for keeping one of said Wiresvibrating at its natural frequency and having an output signalalternating at said one natural frequency, second electronic means forkeeping the other of said wires vibrating at its natural frequency andhaving an output signal alternating at said other natural frequency,means for determining the difference between said natural frequencies,said means comprising a synchronous motor having a pair of statorwindings and a pair of rotor windings, electric-a1 connections betweenthe output of said first electronic means and said stator windings,electrical connections between the output of said second electronicmeans and said rotor windings, and phase splitting means interposed insaid connections.

3. In a device of the character described, a pair of spaced supports, amass, a pair of coaxial tensioned wires extending between said mass andsaid supports, first electronic means for keeping one of said wiresvibrating at its natural frequency and having an output signalalternating at said one natural frequency, second electronic means forkeeping the other of said wires vibrating at its natural frequency andhaving an output signal alternating at said other natural frequency,means for determining the difference between said natural frequencies,means for determining the sum of said natural frequencies, means formultiplying an amount controlled by said sum by said difference toobtain a product, and means for adding said product to said differenceto obtain an output indicative of the velocity of the supports in theaxis of said wires, said means for determining the sums of said naturalfrequencies comprising a pair of demodulator means, each supplied with avoltage of reference frequency and a voltage of one of said naturalfrequencies, filter means connected to the output of each of saiddemodulator means, third demodulator means supplied with outputs of saidfilters.

4. In a device of the character described, a pair of spaced supports, amass, a pair of coaxial tensioned wires extending between said mass andsaid supports, first electronic means for keeping one of said wiresvibrating at its natural frequency and having an output signalalternating at said one natural frequency, second electronic means forkeeping the other of said wires vibrating at its natural frequency andhaving an output signal alternating at said other natural frequency,means for determining the difference between said nautr-al frequencies,means for determining the sum of said natural frequencies, means formultiplying an amount controlled by said sum by said difference toobtain a product, :and means for adding said product to said differenceto obtain an output indicative of the velocity of the supports in theaxis of said wires, said means for multiplying an amount controlled bysaid sum by said difference to obtain a product comprising a discdisplaced according to the difference between said natural frequencies,a ball displaced along said disc according to said amount, a rollerdriven by said ball, the displacement of said roller being proportionalto said product.

5. In a device of the character described, .a pair of spaced supports, amass, a pair of coaxial tensioned wires extending between said mass andsaid supports, first electronic means for keeping one of said wiresvibrating at its natural frequency and having an output signalalternating at said one natural frequency, second electronic means forkeeping the other of said wires vibrating at its natural frequency andhaving an output signal alternating at said other natural frequency,means for determining the difference between said natural frequencies,means for determining the sum of said natural frequencies, means formultiplying an amount controlled by said sum by said difference toobtain a product, and means for adding said output to said difference toobtain an output indicative of the velocity of the supports in the axisof said wires, said means for determining the sums of said naturalfrequencies comprising a pair of demodulator means, each supplied with avoltage of reference frequency and a voltage of one of said naturalfrequencies, filter means connected to the output of each of saiddemodulator means, third demodulator means supplied with outputs of saidfilters,

said means for multiplying amount controlled by said sum by saiddifference to obtain a product comprising a disc displaced according tothe difference between said natural frequencies, a ball displaced alongsaid disc according to said amount, a roller driven by said ball, thedisplacement of said roller being proportional to said product.

6. In a device of the character described, a pair of spaced supports, amass, a pair of coaxial tensioned members extending between said massand said supports, first electronic means for keeping one of saidmembers vibrating at its natural frequency, second electronic means forkeeping the other of said members vibrating at its natural frequency,means for determining the difference between said natural frequencies,said members comprising tapes of rectangular cross section attached tosaid mass so that their wide surfaces lie in perpendicular planes, saidfirst and second electronic means being positioned to vibrate said tapesin the planes perpendicular to their surfaces.

7. In a device of the character described, a pair of spaced supports, amass, a pair of coaxial tensioned members extending between said massand said supports, first electronic means for keeping one of saidmembers vibrating at its natural frequency, second electronic means forkeeping the other of said members vibrating at its natural frequency,means for determining the difference between said natural frequencies,said mass comprising a pair of spaced weights connected by an elastic,non-rigid member aligned with said coaxial tensioned members, fordecoupling said tension members.

References Cited UNITED STATES PATENTS 1,995,305 3/1935 Hayes 26412,591,921 4/1952 Cosgriff et al. 264l 2,697,594 12/1954 Stanton 26412,725,492 11/1955 Allan 264-1 2,762,221 9/1956 Lundquist 264-l FOREIGNPATENTS 729,894 12/ 1942 Germany.

RICHARD C. QUEISSER, Primary Examiner.

SAMUEL BOYD, Examiner.

W. J. CURRAN, R. F. STAHL, JAMES J. GILL,

Assistant Examiners.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,332,290 July 25, 1967 Charles D. Bock et a1.

error appears in the above numbered pat- It is hereby certified that tthe said Letters Patent should read as ent requiring correction and thacorrected below.

Column 4, line 68, for "votlage" read voltage column 7, line 14, for"nautral" read natural line 40, for "output" read product Signed andsealed this 18th day of June 1968.

(SEAL) Attest:

EDWARD J. BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer

1. IN A DEVICE OF THE CHARACTER DESCRIBED, A PAIR OF SPACED SUPPORTS, A MASS, A PAIR OF COAXIAL TENSIONED WIRES EXTENDING BETWEEN SAID MASS AND SAID SUPPORTS, FIRST ELECTRONIC MEANS FOR KEEPING ONE OF SAID WIRES VIBRATING AT ITS NATURAL FREQUENCY AND HAVING AN OUTPUT SIGNAL ALTERNATING AT SAID ONE NATURAL FREQUENCY, SECOND ELECTRONIC MEANS FOR KEEPING THE OTHER OF SAID WIRES VIBRATING AT ITS NATURAL FREQUENCY AND HAVING AN OUTPUT SIGNAL ALTERNATING AT SAID OTHER NATURAL FREQUENCY, MEANS FOR DETERMINING THE DIFFERENCE BETWEEN SAID NATURAL FREQUENCIES, MEANS FOR DETERMINING THE SUM OF SAID NATURAL FREQUENCIES, MEANS FOR MULTIPLYING AN AMOUNT CONTROLLED BY SAID SUM BY SAID DIFFERENCE TO OBTAIN A PRODUCT, AND MEANS FOR ADDING SAID PRODUCT TO SAID DIFFERENCE TO OBTAIN AN OUTPUT INDICATIVE OF THE VELOCITY OF THE SUPPORTS IN THE AXIS OF SAID WIRES. 